The present invention relates to a stent that is implanted in lumens such as the blood vessel, the bile duct, the trachea, the esophagus, the ureter, and the like so that it is used to improve a stenosed portion or a dosed portion generated in the lumens.
To cure various diseases that are caused when the blood vessel or other lumens are stenosed or dosed, the stent which is a tubular-shaped medical appliance generally is implanted at the stenosed portion or the dosed portion to expand them and secure the lumen thereof.
Because the stent is inserted into the body from outside, its diameter is small. The stent is dilated to make-its diameter large at the stenosed or dosed portions to keep the dilated state of the lumen.
The stent is classified into a self-expandable stent and a balloon expandable stent, depending on the function thereof and an implantation method. The balloon expandable stent which itself has no dilating function is secured at a desired portion. Then, a balloon provided in the stent is inflated to dilate (plastically deform) the stent so that the stent comes in dose contact with the inner surface of the desired lumen. That is, it is necessary to dilate the stent of this type in implanting it to the desired portion.
Balloon expandable stents are disclosed in Examined Japanese Patent Publication No. 4-6377 and Japanese Patent Application Laid-Open No. 2-174859. These stents are pipes having axial slots formed therein. The slots are arranged such that they may take inter-connected rhombus shapes when the stent is dilated.
The stent disclosed in Examined Japanese Patent Publication No. 4-6377 is superior in shape retention after dilation because slots take inter-connected rhombus shapes. That is, the stent is resistant to the contracting force of a blood vessel. In other words, the stent has a strong shape-holding force. Another advantage of the stent is that when increase in the diameter of the stent is desired in a dilated state, an additional balloon having an enlarged diameter may be inserted into the stent. The stent of this type is called tube type. The word tube type is derived from the fact that normally, the stent is manufactured by boring a hole on a metal tube (metal pipe) with a laser. The stent of this type has a structural characteristic that it consists of a large number of narrow members crossing each other.
In the stent disclosed in Examined Japanese Patent Publication No. 4-6377, supposing that one segment thereof consists of a long and narrow rectangle, one segment consists of long and narrow members crossing each other at six points. By dilating the long and narrow rectangle, the rectangle plastically deforms into a rhombic shape and maintains the deformed shape, thus being resistant to the contraction force of the blood vessel. The stent of the tube type has various shapes, in addition to the above configuration. But the stent of the tube type has a large number of cross points commonly, irrespective of configurations.
The stent disclosed in Japanese Patent Application Laid-Open No. 1-145076 is called coil-type stent consisting of one wire. The wire is deformed zigzag and wound spirally to form it into a cylindrical shape. Thus, the stent does not have any crossing points of wires.
When for example, the tube-type stent is implanted in a branched blood vessel, the long and narrow members are present at the branch portion, and the branch portion is always subjected to a blood stream. Thus, the tube-type stent may cause thrombus to arise at the branch portion. Another problem of the tube-type stent is that when the branch portion is stenosed again, it is impossible to expand the stenosed portion with a balloon or embed the stent therein, because the long and narrow member interferes with an operation.
On the other hand, the coil-type stent does not have crossing portions of wires. Therefore, it is possible to perform an operation of dilating the branch portion with the balloon. However, the coil-type stent does not have crossing portions of wires. Thus, it has a low force of keeping a dilated state. To increase the force of keeping a dilated state, it is necessary to thicken the wire. As a result, it is not easy to deliver the stent. Further, when the stent is caught by a blood vessel, the shape of the wire becomes out of order. That is, the coil-type stent is inferior to the tube-type stent in its function.